How to Use Readonly Typescript Type

In TypeScript, Readonly is a utility type that makes all properties of an object read-only. This means its properties cannot be changed once an object is created. It's a great way to ensure that your objects remain constant throughout the program, adding predictability and reducing side effects.

Why Use Readonly?

Using Readonly can help in maintaining the integrity of your data. It prevents accidental modifications, which is particularly useful in large codebases where tracking changes can be challenging. Moreover, it clearly communicates the developer's intent that the object should not be modified.

How to Use Readonly

Let's look at a simple example to understand how Readonly works in TypeScript.

interface User {
    name: string;
    age: number;

// Creating a readonly user object
const readonlyUser: Readonly<User> = {
    name: "Niall",
    age: 32

// Trying to modify the object will result in a TypeScript error
readonlyUser.age = 31; // Error: Cannot assign to 'age' because it is a read-only property.

In this example, we define an interface User with name and age properties. We then create an object readonlyUser with the Readonly type applied to User. Any attempt to modify the properties of readonlyUser will result in a TypeScript error, ensuring the object's immutability.

The Readonly type in TypeScript is a simple yet powerful tool to make your objects immutable. More than catching bugs, I like that It clarifies your intent to other developers.

Remember, TypeScript's features are there to help you write better code, and understanding them is key to becoming a proficient TypeScript developer. ✌️

Avatar for Niall Maher

Written by Niall Maher

Founder of Codú - The web developer community! I've worked in nearly every corner of technology businesses; Lead Developer, Software Architect, Product Manager, CTO and now happily a Founder.


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